As energy prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future. Thus, research into various power generation techniques such as nuclear energy, solar energy, wind energy, tidal power, and the like, is underway, and power storage devices for more efficient use of the generated energy are also drawing much attention.
Specifically, demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and demand therefor continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized and the market for lithium secondary batteries continues to expand to applications such as auxiliary power suppliers through smart-grid technology
Since lithium secondary batteries used as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) must have high energy density and exhibit high output within short time, and must be used for 10 years or more under a severe condition that charge and discharge due to large current are repeated in a short time, stability and long-term lifespan characteristics dramatically superior to conventional small lithium secondary battery are necessarily required.
In addition, since lithium secondary batteries used as a high-capacity power storage device must have high energy density and efficiency, and long lifespan, and ignition and explosion due to system malfunction caused by high performance and high capacity are linked to big accident, it is particularly important to secure stability and reliability.
In this regard, as a negative electrode active material of a negative electrode of conventional lithium secondary batteries, carbon based compounds, in which reversible intercalation and elimination of lithium ions are possible while maintaining structural and electrical properties, were mainly used.
In particular, carbon based compounds have an extremely low discharge potential, namely, approximately −3 V, with respect to a standard hydrogen electrode potential and exhibit extremely reversible charge and discharge behavior due to uniaxial orientation of a graphene layer, and, thus, superior electrode cycle life characteristics are exhibited. In addition, since an electrode potential may be 0 V Li/Li+, which is similar to that of pure lithium metal during charge of Li ions, higher energy is advantageously obtained when an oxide based positive electrode and a battery are constituted.
However, negative electrodes composed of such carbon based compounds have problems as follows.
First, since the carbon based compounds have a theoretical maximum capacity of 372 mAh/g, there is a limitation in capacity increase. Thus, there is a limitation in performance of a sufficient role as an energy source of fast changing next-generation mobile devices.
Secondly, since the carbon based compounds exhibit a chemical potential similar to metal lithium when lithium ions are intercalated and eliminated, lithium precipitation due to overpotential is caused even at slightly high charge current and precipitation of lithium, which is precipitated once, is more accelerated as charge and discharge are repeated. Accordingly, capacity reduction and short circuit through dendrites are caused and, thus, stability may be considerably affected.
Thirdly, when lithium larger than an acceptable amount of a negative electrode due to overcharge and the like of a battery is charged, temperature is elevated and an exothermic reaction is caused. Such a reaction is the earliest reaction of thermodynamic reactions occurring within a battery and may be a major trigger of ignition explosion and the like of a battery.
Fourthly, a surface of a hydrophobic electrode greatly affects electrode wetting of an electrolyte solution when a battery is produced and the electrode wetting affects productivity reduction of a battery.
To resolve the problems, a research into negative electrode materials, in which Li alloy reaction using silicone (Si), germanium (Ge), tin (Sn), or aluminum (Al) rather than conventional carbon based negative electrodes is performed, is actively conducted. In the case of silicone as one of such alloy based negative electrode materials, a theoretical maximum capacity is approximately 3580 mAh/g, which is much higher than those of graphite based materials. Tin also has a large capacity of 990 mAh/g.
However, since such alloy based negative electrode materials exhibit very poor lifespan characteristics due to large volume change when charged and discharged, use thereof is greatly limited and, thus, use thereof may be limited.
Meanwhile, other than alloy based negative electrode materials, transition metal oxide based negative electrode active materials composed of a transition metal are also a focus of attention. Vanadium oxides and lithium vanadate based materials as the transition metal oxide based negative electrode active materials are researched as a positive electrode material and negative electrode material of lithium secondary batteries.
As vanadium based materials researched as a positive electrode active material, there are V2O5, LiV3O8, and the like. However, the materials have too low reaction voltage to use as a positive electrode active material of lithium secondary batteries. In addition, since lithium metal is used as a negative electrode, it is difficult to use the materials as a positive electrode material of actual batteries. At present, as positive electrode materials, a research into lithium vanadium phosphate, lithium vanadium fluoride phosphate, and the like, in which reaction voltage using inductive effects of multiple anion based materials is increased, is actively performed.
Meanwhile, research into vanadium based materials as a negative electrode active material of lithium secondary batteries using low reaction voltage of the vanadium based materials is performed. As representative materials, there are V2O5 and LiVO2 based materials (for example, Li1−xVO2, Li1−xV1+xO2), LiMVO4, where M is Zn, Cd, Co, or Ni, MV2O6+δ, where M is Fe, Mn, or Co, and the like, which may be used as a positive electrode active material.
For example, in J. Electrochem. Soc., Vol. 154, pp. A692-A697, L. Cheng et al., 2007, it was reported that reversible phase transition was caused when crystalline V2O5 stored one lithium or more within a structure thereof.
Therefore, since a structure of crystalline V2O5 is destroyed as cycles proceeds and capacity is decreased, it is disadvantageous to use the crystalline V2O5 as a negative electrode active material. To improve such problems, research into synthesis of amorphous V2O5 is underway.
In addition, in Mater. Chem. Phys. Vol. 116, pp. 603-606, N. S. Choi, et al., 2009, it was reported that Li1−xV1+xO2 based materials having the same layered structure as LiCoO2 most commonly used as a positive electrode active material expressed a capacity in a voltage section of 0.5 V lower than lithium and, thus, had operation voltage similar to a lithium secondary battery utilizing the commonly used carbon-based negative electrode active materials, when the Li1−xV1+xO2 based materials were assembled into a lithium secondary battery.
However, the material has a significant limitation in that a reversible capacity thereof is approximately 200 mAh/g, much lower than a capacity of graphite. In addition, such a low reversible capacity is considered as being caused by vanadium, which already trivalently exists in a Li1−xV1+xO2 material, stores lithium and additional reduction thereof is difficult.
In addition, in Solid State Ionics, Vol. 107, pp. 123-133, F. Orsini et al., 1998 and J. Power Sources., Vol. 68, pp. 698-703, Y. Piffard et al., 1997, it was reported that LiMVO4 and MV2O6+δ may express dramatically high capacity, when compared with the vanadium based negative electrode active materials listed above.
However, in J. Electrochem. Soc., Vol. 148, pp. A869-A877, C. Rossignol et al., 2001 and Solid State Ionics, Vol. 139, pp. 57-65, S. S. Kim et al., 2001, it was reported that such high capacity is a phenomenon greatly helped and expressed by changes in the oxidation number of vanadium and the oxidation numbers of other transition metals, where M is Ni or Mn, included in the material.
In addition, some prior technologies disclose methods of adding small amounts of some vanadium oxide types to carbon based compounds to resolve the above problems. However, such vanadium oxides added in small amounts are recognized as inactive materials which do not store lithium in carbon based compounds and, thus, capacity in a low voltage section is partially increased by increasing a diffusion rate of lithium ions and there is fundamental limitation due to low reversible capacity of the carbon based compounds.
Therefore, there is an urgent need to develop a negative electrode, which may be used as a high-capacity power source, having improved stability, high-output characteristics and high energy density.